This is a basic tutorial on how to use Rigid Bodies and Physics Materials to make objects fall and bounce in our Unity projects.
Video Transcript
Hello everyone!
This is a basic tutorial on how to use Rigid Bodies and Physics Materials to make objects fall and bounce in our Unity projects.
Unity provides a physics simulation engine, but in order to access these features, it is necessary to use a couple of components (such as Rigid Body and Colliders) and other elements.
This is because physical simulation is computationally expensive, so by default, objects do NOT participate in it. It's up to us to specify if and how an object should behave physically.
Okay, let's get started!
I am setting up a simple scene in Unity with a Cube as the floor and a basketball model.
We immediately notice that the Cube object, which is a native Unity object, already has a Box Collider component, while my 3D model, which I created from an external file, does not have any Collider component of any kind.
We'll come back to this later.
I positioned the basketball model right above the Cube, at a certain distance, but when I press the Play button, I immediately notice that the ball... does not fall.
To make the ball participate in the physical simulation of the scene, we just need to provide it with a Rigid Body component, which can be easily added in the object's Inspector.
In the component's tab, we notice some interesting features; among them, the mass (which we can leave as is for this example) and the "Use Gravity" checkbox, which is selected by default, so this time we can start the game and notice that, in fact, the ball falls...
... but passes through the underlying cube, falling into the void!
You might think of adding a Rigid Body component, possibly without gravity, to the underlying Cube object as well, to make both objects participate in the physical simulation... however, we can immediately verify that this assumption does not solve the problem (so, remove the Rigid Body from the cube, to avoid confusion).
Simply put, the ball passes through the cube... yet the Cube has a Box Collider, so it should detect the collision...
The fact is that it's the basketball model that does NOT have its own Collider, so we need to provide it with one in the Inspector. Since the basketball has a spherical shape, we choose a Sphere Collider for this object.
Starting the simulation now, we will see the ball fall onto the Cube and remain still on it.
So, keep in mind that even the Cube (and, in general, any scene object that will interact with the ball) must have its own Collider!
So, to recap: in order to interact with each other, two scene objects must both have Colliders, otherwise they will behave like ghosts; furthermore, in order to participate in the physical simulation (and thus be affected by gravity and other forces), an object must also have a Rigid Body component.
However, the result achieved so far is not satisfactory: the ball falls and impacts the underlying cube, but it does not bounce!
The properties of the object's material, however, should not be assigned to the Rigid Body, but to the Collider: these properties concern ANY scene object, not just those that participate in the physical simulation (i.e., Rigid Bodies), so we need to set them in the Collider tab.
In this tab, we find a field called Material, which is set to None by default but suggests that we need to provide it with a "Physic Material" object.
So let's add a Physic Material object to the Project; attention: "Physic Material", not simply "Material", which instead concerns the visual aspect of an object.
We then assign this new Physic Material to the basketball's Sphere Collider, press Play, and... notice that the ball does not bounce.
This is because, by default, the bounciness of a new Physic Material is set to 0, as we can see by selecting such object and observing its few properties in the Inspector.
The Physic Material has two main properties: "Bounciness" and "Friction," which can be adjusted to create a more or less realistic bouncing behavior; for example, increasing the "Bounciness" can increase the force of the bounce, while increasing friction can decrease the distance traveled after each bounce.
So let's try setting Bounciness to 1 (maximum value for this field) and start the game: this time, the ball bounces a couple of times before stopping!
The behavior is realistic, but how can we make the ball bounce more?
We can act, for example, on the Friction parameters and, above all, on the two Combine parameters: these parameters determine how to combine the physical parameters of two objects when they come into contact.
The default value for Bounce Combine is Average, so it will average between the basketball (for which we set Bounciness to 1) and the underlying cube, which does not have a Physics Material and will therefore be seen by Unity as 0, so the final Bounce value will actually be 0.5.
If we change Bounce Combine to Maximum, this time the actual Bounciness in play will be 1 and the ball will bounce much more...
... but now the problem is that it will keep bounce more!
It's actually a strange behavior; apparently, to maintain constant bounces, the Bounciness value must be 0.965... don't ask me why.
Obviously, in simulating bounces, the surface is also important: so we need to provide a Physic Material to the Cube (but the same goes for floors, walls, and other objects made of different materials and therefore with different physical properties) and set its Bounciness appropriately...
… but I leave this to you: try various combinations, with different values for Bounciness, Friction, and Combine!
That's it for this tutorial! See you soon!